1. Field of the Invention
The present invention relates to single-ended to differential converters.
More particularly, the present invention relates to a circuit technique to perform attenuation and single-ended to differential conversion with common-mode voltage control on a single integrated circuit (IC).
2. Background Art
Advances in electronic device packaging provide electronic system designers with the ability to include an increased number of functions on a single IC. Such ICs are particularly well suited for low supply voltage applications, especially those ICs that can accommodate processing of analog signals. Thus, by combining numerous functions on a single IC, electronic system designers are able to realize tremendous savings in power consumption and space. Even greater functional advantages can be realized through the selection of particular signal processing techniques to be used on these ICs.
For example, in the case of analog signals on single ICs, differential signal processing is preferred over single-ended processing because it provides better rejection of power supply and substrate noises. Differential signal processing is also inherently better at rejecting common-mode voltages than single-ended processing. For purposes of illustration, differential signal processing is commonly used in single-chip video receivers, also known as on-chip receivers. In on-chip receivers, however, certain applications often benefit more from the input of single-ended signals instead of differential signals. Also, from the standpoint of hardware, single-ended signals may be preferred as circuit inputs because they are easier to provide as circuit inputs and can be more cheaply produced than their differential-ended versions. When single-ended signals are provided as circuit inputs, conventional on-chip receivers normally include additional on-chip circuitry to convert the received single-ended signals into differential signals in order to facilitate the more advantageous differential signal processing.
A number of traditional techniques exist to convert single-ended signals to differential signals. One popular technique includes converting the single-ended signal from the voltage domain to the current domain. With this particular technique, the single-ended signal is received at one of the inputs of a voltage to current converter. In response, the converter produces at its output, a differential signal, having a positive signal component and a negative signal component. Both signal components, however, are in the current domain. To convert this differential current signal from current domain to voltage domain, the differential signal is forwarded to a current to voltage converter, such as resistor, in order to finally create the differential voltage signal. One deficiency with this approach is that it""s not well suited for low voltage power supplies. More specifically, it""s difficult to fit this approach within the voltage head-room constraints of low voltage power supplies.
Another technique for converting single-ended signals to differential signals includes the use of a degenerate differential pair. However, when providing one side of a degenerate differential pair with a relatively large signal, while at the same time, leaving the other side at a constant voltage, its extremely difficult to achieve acceptable linearity. Providing one side of the degenerate differential pair with a large signal and leaving the other side at a constant voltage is required in order to convert a single-ended signal to a differential signal.
In addition to single-ended to differential converters, attenuators are also circuits that are commonly used in on-chip receivers. When used to attenuate input signals, the dynamic range of attenuators can be adjusted to fit within the dynamic range of subsequent signal processing blocks, such as track and hold circuits or analog-to-digital converters (ADCs).
Although numerous traditional techniques exist to convert single-ended signals into differential signals, low voltage power supplies often do not provide adequate voltage head-room to accommodate efficient use of these traditional techniques. As a result, there is a need for a device that can provide and improved approach for converting a single-ended signal to a differential signal without the deficiencies of the traditional approaches discussed above. Also, there is a need to provide an improved single-ended to differential conversion technique and an attenuator on a single IC chip to save power and optimize circuit board space.
Consistent with the principles of the present invention as embodied and broadly described herein, an exemplary circuit includes an attenuator having first and second receiving ports configured to respectively receive first and second input signals and differential output ports configured to output a differential signal produced in accordance with the first and second input signals. The attenuator also includes an adjusting node positioned in association with the differential output ports. A stabilizing circuit, including a sensor and a comparator, is configured to receive the differential signal. The sensor is coupled across the differential output ports and has a feedback node. The comparator includes two input ports and a comparator output port. A first of the two input ports is coupled to the feedback node and the other of the two input ports is configured to receive a reference voltage signal. The comparator output port is coupled to the adjusting node. The sensor is configured to detect a common-mode voltage level of the differential signal and provide the detected common-mode voltage level to the first input port. The comparator is configured to compare the common-mode voltage level to a level of the reference voltage signal and produce an adjusting voltage signal based upon the comparison. Finally, the adjusting voltage signal is applied to the adjusting node and is operative to adjust a subsequent common-mode voltage level of the differential signal.
Features and advantages of the present invention include the ability to beneficially combine the functions of a low power single-ended to differential converter and an attenuator on a single IC. This approach increases the performance and provides additional space on the IC for incorporation of added functions. Additional advantages of the present invention are the ability to provide single-ended to differential conversion in a manner that fits within the voltage head-room constraints of low voltage power supplies and a provision for controlling common-mode voltage in an efficient manner.